The present invention relates generally to cutting inserts and, more particularly, to threading inserts having at least an entire face formed of PCBN.
“Hard turning” operations are typically considered those involving material having hardness 45 HRC and up. When manufacturing components produced in hard materials such as hardened steels and hard chilled/white cast irons (45 HRC and up) there are occasions where heavy duty thread forms are required to be machined into the components. These thread forms are Trapezoidal in shape. For purposes of the present application, “Trapezoidal” threads shall be defined as threads with 29° and 30° included angles. They may have asymmetric thread forms. Types of these thread forms include Acme, Stub Acme, ISO TR-DIN 103, Buttress, and Dardelet threads. These thread forms have a stronger thread profile than standard threads which makes them particularly useful in translational applications such as power transmission applications involving moving heavy machine loads as found on machine tools, slurry pumps etc. As the finished components are of a high hardness, there are currently two established methods employed to produce the Trapezoidal thread form.
The most common method of forming the Trapezoidal thread form is to machine the thread form while the material is in its unhardened soft state with tungsten carbide tooling. In the case of chilled/white cast iron, this can mean annealing it several times (e.g., up to 3 times) in order to make the chilled/white iron soft enough to allow tungsten carbide tooling to complete the desired thread form. Once the thread form is produced with tungsten carbide thread profile tools, the components are then heat treated to bring them up to the required hardness (up to 700 HBN). As the hardening process can induce distortion, it is common for the thread to be chased very slowly with tungsten carbide threading tools/inserts. In the case of chilled/white cast iron machining, even after triple annealing, the material is still sufficiently hard and abrasive to limit tungsten carbide tooling to slow cutting speeds (typically 50 m/min) and relatively short tools life. In addition the annealing process is costly in both time and energy costs. For both chilled/white cast iron and hard steels, the whole process is costly and time consuming and the thread form can be the major machining cost for the entire component.
Another method for forming the Trapezoidal thread form centers on a cutting tool material called Polycrystalline Cubic Boron Nitride (PCBN). PCBN is able to machine these materials in their hardened state at relatively high cutting speeds and feeds and can reduce the costs and time associated with cutting heavy duty Trapezoidal threads in hardened steel and hard chilled/white cast iron components. Being able to machine the thread directly into the hardened component allows for all annealing processes and the post thread chasing process to be eliminated from the machining cycle, thereby significantly reducing cycle times and production costs associated with producing the thread. Currently, manufacturers of PCBN Trapezoidal threading tools braze a piece of PCBN 1 or, more typically, a piece of PCBN attached to a substrate such as tungsten carbide, to a mild steel shank 2 and grind the Trapezoidal shape to the given thread form to form a PCBN brazed shank cutting tool as seen in FIGS. 1A and 1B. A braze joint 3 is disposed between the PCBN 1 and the shank 2.
The method of using a PCBN tip brazed to a mild steel shank, while offering benefits over using tungsten carbide threading inserts with the workpiece in its softened state, also has several drawbacks. These limitations relate largely to the bonding of the PCBN tip to the mild steel shank as well as to the braze joint that bonds the PCBN tip to the mild steel shank body. For example, when machining with PCBN cutting tools, heat generation is an important factor in the cutting process. Cutting speeds for brazed PCBN shank tools have to be limited in order to stop the braze joint from heating up and failing. The result is that the tool life and productivity is restricted.
Also, generally, brazed PCBN shank tools are run with coolant in order to keep the braze joint cool during machining. Running brazed PCBN shank tools without coolant has been shown to result in braze failure and therefore, catastrophic failure of the threading tool.
Because, with brazed PCBN shank tools, cutting speed is limited, the lack of heat generation when running at lower cutting speeds results in the development of a built up edge (BUE). This is when the material being cut welds to the cutting edge. This is detrimental to tool life as eventually the BUE breaks off, typically taking with it part of the cutting edge.
With brazed PCBN shank tools, the worn brazed PCBN tip is removed after each thread and datums have to be set for each new thread, which is time consuming and costly. Additionally, in brazed PCBN shank tools, cutting edges are typically produced with a tool and cutter grinding machine, which can result in a short tool life.
It is desirable to provide a cutting tool and method with which heavy duty threads can be formed in hardened materials at low cost and high speed.
According to an aspect of the present invention, a threading insert comprises a cutting edge, an insert supporting surface and a clamping surface for clamping the insert relative to a toolholder such that the insert supporting surface supports the insert against an abutment surface of the toolholder, and at least one layer of PCBN forming a face of the insert, the cutting edge extending at least partially around a periphery of the insert and being formed in the PCBN layer.
According to another aspect of the present invention, a threading tool comprises a toolholder comprising a recess, the recess comprising an abutment surface, and a threading insert receivable in the recess, the threading insert comprising a cutting edge, an insert supporting surface and a clamping surface for clamping the insert relative to the toolholder such that the insert supporting surface supports the insert against the abutment surface, and at least one layer of PCBN forming a face of the insert, the cutting edge extending at least partially around a periphery of the insert and being formed in the PCBN layer.
According to yet another aspect of the present invention, a method of threading a workpiece comprises clamping a threading insert to a toolholder with a clamp, the insert comprising an insert supporting surface and a clamping surface for clamping the insert relative to the toolholder such that the insert supporting surface supports the insert against an abutment surface of the toolholder, and at least one layer of PCBN forming a face of the insert, a cutting edge extending at least partially around a periphery of the insert and being formed in the PCBN layer. A workpiece and the insert are moved relative to each other, and the workpiece is contacted with the cutting edge.